1. Field of the Invention
The present invention relates to integrated circuit packages. More particularly, the present invention relates to a method and apparatus for socketing semiconductor devices and attaching semiconductor sockets to printed circuit boards and similar structures.
2. The Prior Art
The package for an integrated circuit ("IC") semiconductor device typically fulfills three functions. It provides environmental and physical protection for the silicon chip, it provides a means to facilitate handling of the chip, and it provides electrical connection from the chip to the system in which it is installed. Packages are usually soldered to their circuit boards to physically and electrically connect the package to the circuit board.
An additional level of packaging--a socket--is sometimes used to electrically and physically connect the IC package to its printed circuit board. Sockets for IC packages are usually fitted with pins which are soldered to a circuit board; pressure contacts can also be used, in which case the socket is pushed against the circuit board with screws, springs, or some other type of mounting hardware. In the latter configuration pressure contacts must have enough compliance or conformance to compensate for non-planarities present in the circuit board and/or the mounting surface of the IC package.
With the increasing cost and complexity of some integrated circuits and the high penalties often associated with equipment down-time, there is an increasing need for component repairability and an ability to change system functionality with little or no impact on the rest of the system. It is well understood that determination of a defect's cause is made easier by minimizing perturbations of the defective system. Removing a device from its conventional socket may cause changes in the socket/IC package system which will prevent adequate failure analysis. While allowing the removal and replacement of components, neither type of socket described above can be removed from a circuit board easily. Thus, a defective part can be easily replaced, but not a defective socket. Also, the time it takes to determine the cause of a failure--whether in the device itself or in the socket--may result in an unacceptable amount of down-time.
There are numerous examples of the first type of socket described above sold to various standard through-hole and surface mount footprints by a variety of vendors. Nepenthe Distribution, Inc. (U.S.A.) of Palo Alto, Cal., for example, sells sockets for various pin grid arrays and quad flatpacks such as the 225-pin PGA socket (Part Number NEPS-225-RS15) and the 68-pin PLCC socket (Part Number 1-068-0000). Both of these are intended to be soldered to a circuit board. U.S. Pat. No. 5,176,524 to Iseki et al. entitled "IC Socket Structure" teaches the use of sockets for surface mount devices ("SMDs") that are themselves surface mountable. These are also intended for soldering to a printed circuit board.
Due to the fact that both through-hole and surface mounted sockets are permanently attached to their circuit boards by solder, repair and replacement of such sockets from their circuit boards is a serious problem, usually requiring removal of the circuit board for a time from its system. The socket must be removed by localized or general heating of the circuit board past the melting point of solder. Further cleaning and preparation are then required before a new socket can be installed on the circuit board. Such delays and the accompanying potential for additional damage to expensive circuit boards due to heating makes these types of sockets unsuitable for many applications.
Of the second type of socket, U.S. Pat. No. 5,127,837 to Hopfer et al. entitled "Electrical Connectors and IC Chip Tester Embodying Same" teaches the use of an insulator ring permanently affixed to a printed circuit board wherein the insulator ring is fitted with resilient wadded conductors which provide electrical connection from an integrated circuit package to a circuit board. In this case, the socket is not soldered to the circuit board, but rather is attached by mounting hardware such as screws and is not adapted to be removed from the circuit board without the IC package first being removed from the socket. Thus the socket and IC do not form an integral package.
Similarly, U.S. Pat. No. 4,692,790 to Oyamada entitled "Structure for Connecting Leadless Chip Carrier" teaches a means of aligning an IC package to a printed circuit board and use of an elastomeric interconnect medium. A hinged cover with a spring in its center applies a downward force against the IC package, creating an electrical connection between the IC package and the circuit board. Although the interconnect medium and the IC package can be replaced, there are no provisions for removing the entire socket from the circuit board. Other sockets suffer from the same drawbacks. See, e.g., U.S. Pat. No. 5,100,332 to Egawa entitled "IC Socket" and U.S. Pat. No. 4,954,088 to Tsutomu et al. entitled "Socket for Mounting an IC Chip Package on a Printed Circuit Board".
U.S. Pat. No. 5,161,983 to Ohno et al. entitled "Low Profile Socket Connector" teaches attaching a sheet of elastomer to a molded plastic base by means of plastic pegs. Several problems are presented by such a structure. Elastomers as described are prone to tearing, particularly at stress concentrations such as those applied when stretching elastomers over pegs. Further, elastomers typically require low pressures to engage electrically two opposing point contacts. Localized deformation of the elastomer--typically a low-to-medium durometer silicon rubber--enable such low contact force engagements. Over larger areas, such as those found across land-grid array ("LGA") packages, there is no localized deformation of the elastomer. In such cases the bulk elastomer material must be compressed, requiring significantly higher forces to engage electrically the contacts on either side of the elastomer. Although this patent teaches contact on LGAs with three rows of contacts on each of the four sides of the LGA's bottom surface, substantial force would need to be applied to the LGA to compress the bulk elastomer material enough to create electrical connection between the LGA and the circuit board given the clamping mechanisms described.
Accordingly, there is a need for a demountable IC socket with high pin density.